test: characterization tests for Flight.step_simulation()

Verify the stepped-simulation API (run_simulation=False plus repeated
step_simulation() calls) reproduces a one-shot simulate():

- initial _step_state; stepping reaches the finished state
- stepped final state (t, y_sol) and the full solution array match simulate()
  to tight tolerance
- post_process_simulation / initialize_prints_plots fire on finish
  (t_final, prints, plots present)
- stepping after finished is a no-op

Uses the parachute-free calisto flight, since parachute triggers are not yet
migrated into the stepping path.

Author: thc1006

tests/unit/simulation/test_step_simulation.py

@@ -0,0 +1,94 @@
+"""Characterization tests for ``Flight.step_simulation()``.
+
+The fork's stepped-simulation API (``run_simulation=False`` plus repeated
+``step_simulation()`` calls) must reproduce the one-shot ``simulate()`` result so
+that callers driving the flight one node at a time -- e.g. real-time control
+loops such as the Balloon Popping Challenge environment -- obtain the same
+trajectory. A parachute-free rocket (``calisto`` in the plain environment) is
+used because parachute triggers are not yet migrated into the stepping path.
+"""
+
+import numpy as np
+
+from rocketpy import Flight
+
+# Standard parachute-free launch, matching the ``flight_calisto`` fixture.
+RAIL_LENGTH = 5.2
+INCLINATION = 85
+HEADING = 0
+
+
+def _make_flight(calisto, example_plain_env, run_simulation):
+    return Flight(
+        environment=example_plain_env,
+        rocket=calisto,
+        rail_length=RAIL_LENGTH,
+        inclination=INCLINATION,
+        heading=HEADING,
+        terminate_on_apogee=False,
+        run_simulation=run_simulation,
+    )
+
+
+def _run_stepped(flight, max_steps=100000):
+    """Drive ``step_simulation`` to completion; return the number of calls."""
+    steps = 0
+    while not flight._step_state["finished"]:
+        flight.step_simulation()
+        steps += 1
+        assert steps < max_steps, "stepped simulation did not terminate"
+    return steps
+
+
+class TestStepSimulation:
+    """Stepping must match a one-shot ``simulate()`` and finalise correctly."""
+
+    def test_initial_step_state(self, calisto, example_plain_env):
+        flight = _make_flight(calisto, example_plain_env, run_simulation=False)
+        assert flight._step_state == {
+            "phase_index": 0,
+            "node_index": 0,
+            "phase_initialized": False,
+            "finished": False,
+        }
+
+    def test_stepping_reaches_finished(self, calisto, example_plain_env):
+        flight = _make_flight(calisto, example_plain_env, run_simulation=False)
+        assert flight._step_state["finished"] is False
+        _run_stepped(flight)
+        assert flight._step_state["finished"] is True
+
+    def test_stepped_final_state_matches_simulate(self, calisto, example_plain_env):
+        reference = _make_flight(calisto, example_plain_env, run_simulation=True)
+        stepped = _make_flight(calisto, example_plain_env, run_simulation=False)
+        _run_stepped(stepped)
+        np.testing.assert_allclose(stepped.t, reference.t, rtol=1e-9, atol=1e-9)
+        np.testing.assert_allclose(stepped.y_sol, reference.y_sol, rtol=1e-8, atol=1e-9)
+
+    def test_stepped_full_solution_matches_simulate(self, calisto, example_plain_env):
+        reference = _make_flight(calisto, example_plain_env, run_simulation=True)
+        stepped = _make_flight(calisto, example_plain_env, run_simulation=False)
+        _run_stepped(stepped)
+        reference_solution = np.array(reference.solution)
+        stepped_solution = np.array(stepped.solution)
+        assert stepped_solution.shape == reference_solution.shape
+        np.testing.assert_allclose(
+            stepped_solution, reference_solution, rtol=1e-8, atol=1e-9
+        )
+
+    def test_post_process_artifacts_after_stepping(self, calisto, example_plain_env):
+        stepped = _make_flight(calisto, example_plain_env, run_simulation=False)
+        _run_stepped(stepped)
+        # post_process_simulation() and initialize_prints_plots() fire on finish.
+        assert stepped.t_final == stepped.t
+        assert stepped.prints is not None
+        assert stepped.plots is not None
+
+    def test_step_after_finished_is_noop(self, calisto, example_plain_env):
+        stepped = _make_flight(calisto, example_plain_env, run_simulation=False)
+        _run_stepped(stepped)
+        t_final = stepped.t
+        y_final = np.array(stepped.y_sol)
+        stepped.step_simulation()  # already finished -> must return immediately
+        assert stepped.t == t_final
+        np.testing.assert_array_equal(stepped.y_sol, y_final)